The present invention relates generally to methods for generating a clock in satellite and/or wireless networks. More specifically, the present invention relates to methods for generating an accurate, Doppler-free local clock in satellite and/or wireless networks.
In a distributed satellite/wireless network based on Time Division Multiple Access (TDMA) technology, terminals need a highly stable local timing source (i.e., clock) to generate TDMA frame timing and to generate terrestrial interface clocks. One option is to require an expensive timing source in every terminal, which is inappropriate for large low-cost networks. A second option, that has been used to date, is to install a highly stable clock at the Reference Terminal (RT) only; traffic terminals (TTs) use an inexpensive voltage controlled oscillator (VCXO) or a direct digital synthesizer (DDS) with a free running, inexpensive oscillator, that is fine-tuned to derive a stable clock. The reference terminal transmits a reference burst once per TDMA frame time. The traffic terminal""s receive frame timing is modified based on arrivals of the reference bursts. Periodically, the traffic terminal transmits a management burst to the reference terminal; the reference terminal reports the error in timing to the traffic terminal, which in turn modifies its transmit frame timing appropriately. The control information for tuning the VCXO or DDS is derived from the timing corrections made to the terminal receive timing. Effectively, the VCXO or DDS is tuned so that the derived local clock is phase locked to the received reference burst arrival rate.
The above method results in a traffic terminal clock that is as stable as the reference terminal clock, over any given large time period. Any drift in the local oscillator is automatically removed. However, the traffic terminal clock includes the Doppler frequency caused by the daily movement of the satellite, which is caused due to orbit imperfections. Hence, in any 24-hour period, the derived clock rate will vary by xc2x1D, where D is the maximum change in clock rate caused by Doppler. Several different approaches have been proposed to correct the local clock with respect to a precision clock. For example, U.S. Pat. No. 4,602,375 discloses a procedure for correcting a clock onboard a satellite using drift prediction. In contrast, U.S. Pat. No. 4,639,680 discloses the use of an average of phase error signals in determining an appropriate clock correction value. Both U.S. Pat. Nos. 4,602,375 and 4,639,680 are incorporated herein by reference for all purposes.
A satellite/wireless TDMA system requires an accurate local clock at each traffic terminal to transmit and receive bursts in synchronism with the TDMA frame timing established by a reference terminal. The transmit and receive frame counters are hardware counters that repeatedly count from 0 to Nxe2x88x921 and are clocked by the local clock. N is the length of the TDMA frame expressed in units of the clock cycle time. The transmit frame counter is used to position transmit bursts within a frame; the receive frame counter is used to position an xe2x80x9caperturexe2x80x9d around the expected receive time of every burst.
Differences in the actual rates of the clocks used by the reference terminal and the traffic terminal cause the reference burst to arrive slightly earlier or slightly later than the expected arrival time at the traffic terminal. The traffic terminal measures this time difference for every arriving reference burst and adjusts the local receive frame counter to either extend the next frame time or to shorten the next frame time, as appropriate. This correction is referred to as a Receive Timing Correction (RTC). With this procedure, the receive frame timing of the local terminal xe2x80x9ctracksxe2x80x9d the transmit frame timing of the reference terminal. The rate of these corrections is equal to the difference in frequencies between the local clock and the reference terminal clock; for example, if the frequency difference is n Hz, then the receive timing will be correct, on average, by n units every second.
Another factor that contributes to the frequency difference between the reference terminal and a traffic terminal clocks is the relative satellite motion. As the distance between a terminal and the satellite change, due to imperfections in satellite orbit, the delay between the local terminal and the reference terminal changes. This results in reference bursts arriving earlier or later at the local terminal, which results in corrections to the local receive frame counter. Hence, the rate of change of the receive timing corrections is not just a function of the difference in frequencies between the local clock and the reference terminal clock, but it is also a function of the rate of change of satellite delay (referred to as satellite Doppler).
A similar procedure is used to track the transmit timing of the traffic terminal. The traffic terminal periodically transmits a management burst to the reference station. Differences in the clocks used by the reference terminal and the local terminal cause the management burst to arrive slightly earlier or slightly later than the expected arrival time at the reference terminal. The reference terminal measures this time difference for every arriving management burst and sends the difference value to the traffic terminal. The traffic terminal in turn adjusts the local transmit frame counter to either extend the next frame time or to shorten the next frame time, as appropriate. This correction is referred to as a Transmit Timing Correction (TTC). With this procedure, the transmit frame timing of the traffic terminal xe2x80x9ctracksxe2x80x9d the receive frame timing of the reference terminal. The rate of these corrections is a function of the difference in frequencies between the local clock and the reference terminal clock and the rate of change of satellite delay. Transmit timing correction can also be accomplished by a traffic monitoring its own management burst, if possible, and measuring the timing error.
Traditionally, RTC has been used as a basis for adjusting the frequency of the local oscillator. The rate of receive corrections has been used as a correction factor that is applied to the local oscillator. The local oscillator is adjusted such that the net amount of corrections made to the receive frame counter is zero over any extended period of time. In simple terms, if the net amount of RTCs is positive, the local oscillator frequency is decremented by an appropriate amount; if it is negative, its frequency is incremented by an appropriate amount. The long term stability of the local clock is the same as that of the reference clock; however, the local clock includes the daily variations of satellite Doppler.
Consider a simple example, where the reference terminal clock frequency is R, the traffic terminal clock frequency is also R, the traffic terminal clock is not corrected based on RTC, and the satellite to traffic terminal distance is decreasing at a constant rate. Receive frames will be shortened in time, causing receive timing to be corrected at a rate of d bits/sec, where d is such that the received bit rate appears to be R+d instead of R due to satellite Doppler. Frame timing transmitted by the local terminal using the clock rate R will similarly be shortened at arrival at the reference terminal due to the Doppler. The reference terminal will send transmit timing corrections to the traffic terminal to delay its transmit timing. The rate of timing corrections as seen at the traffic terminal will be xe2x88x92d. Hence, in steady state, the receive timing correction rate will be d and the transmit timing correction rate will be xe2x88x92d.
Now consider the same example except that the local clock frequency has changed to Rxe2x88x92r due to drift in the local oscillator frequency. Assuming no corrections are made to the local clock, the RTC rate will become d+r and the TTC rate will become xe2x88x92d+r.
Thus, if the traditional clock correction algorithm, which is based on RTC rate, is allowed to operate, the local clock rate will change from Rxe2x88x92r to Rxe2x88x92r+d+r, i.e., R+d, the RTC rate will become 0, and the TTC rate will become xe2x88x922d. The net effect is that the local clock drift r is removed but the resultant clock includes the Doppler component.
What is needed is a clock generation algorithm (CGA) which advantageously employs measured timing error information to automatically remove both clock drive and satellite Doppler from the local clock of a traffic terminal. Stated another way, what is needed is a CGA which reproduces an accurate clock at the traffic terminal whose long-term stability matches that of the frequency standard residing at the reference terminal and that is free of satellite Doppler. Moreover, what is needed is a clock that advantageously can be used to provide clocks on terrestrial interfaces, i.e., components coupled to the traffic terminal. Furthermore, it would be desirable if the hardware device required to implement the CGA could be a low-cost component, e.g., a voltage controlled oscillator (VCXO) or a direct digital synthesizer (DDS) coupled to a free running oscillator.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a method for generating an accurate, Doppler-free clock in satellite and/or wireless networks which overcomes the above-described deficiencies. The present invention was motivated by a desire to overcome the drawbacks and shortcomings of the presently available technology, and thereby fulfill this need in the art.
The present invention is a method for producing a Doppler-free local clock Advantageously, this Doppler-free local clock can be used to provide an accurate signal to systems connected to traffic terminals which are not equipped with respective precision clocks.
One object according to the present invention is to provide a method for producing a local clock at each traffic terminal that is free of the frequency offset caused by Doppler due to satellite motion. Advantageously, this Doppler-free local clock can be used to clock out data to terrestrial interfaces without passing satellite Doppler.
Another object according to the present invention is to provide a method for controlling either a low cost VCXO or a low cost DDS with an associated free-running oscillator at traffic terminals to thereby generate a Doppler-free local clock.
Still another object according to the present invention is to provide a method for generating a Doppler-free local clock irrespective of whether the associated network is a single beam (or global beam) TDMA network or a multibeam TDMA network.
Still another object according to the present invention is provide a method for generating a Doppler-free local clock employing an adaptive averaging period, which provides rapid responsiveness when the frequency offsets are relatively large and more precise measurements and corrections when frequency offsets are relatively small.
These and other objects, features and advantages according to the present invention are provided by a method for generating a Doppler-free local clock in a communications network including a master reference terminal and a terminal exchanging reference and management bursts. Advantageously, the method includes steps for determining a transmit timing correction value responsive to the management burst received by the master reference terminal, determining a receive timing correction value responsive to the reference burst received by the terminal, and adjusting the frequency of a clock responsive to both the transmit timing correction value and the receive timing correction value to thereby generate the Doppler-free local clock.
These and other objects, features and advantages according to the present invention are provided by a method for generating a Doppler-free local clock in a communications network including a master reference terminal and a terminal exchanging reference and management bursts. Preferably, the method includes steps for initialing the master reference terminal responsive to a first reference burst generated by the master reference terminal, determining a transmit timing correction value responsive to the management burst received by the master reference terminal, determining a receive timing correction value responsive to a second reference burst received by the terminal, and adjusting the frequency of a clock responsive to both the transmit timing correction value and the receive timing correction value to thereby generate the Doppler-free local clock.
These and other objects, features and advantages according to the present invention are provided by a method for generating a Doppler-free local clock in a communications network including a master reference terminal and a terminal exchanging reference and management bursts Preferably the method includes steps for:
(1) initializing the master reference terminal responsive to a first reference burst generated by the master reference terminal,
(2) determining a transmit timing correction value responsive to the management burst received by the master reference terminal,
(3) determining a receive timing correction value responsive to a second reference burst received by the terminal, and
(4) accumulating the transmit timing correction value and the receive timing correction value to thereby generate a total accumulated error value,
(5) determining whether a frequency adjustment is required responsive to the total accumulated error value,
(6) when the frequency adjustment is not required, repeating the steps (2) and (3),
(7) when the frequency adjustment is required, calculating an adjustment value which is applied to the frequency of a clock responsive to the total accumulated error value to thereby generate the Doppler-free local clock.
According to one aspect of the present invention the calculating step uses the formula
f=(ynxe2x88x92prevyn)/Tc/2+yn/Ty/2
where
yn is the total accumulated error, since the last receive acquisition was successfully performed,
prevyn is the value of yn when the previous clock correction was made,
f is the adjustment value indicative of the required change in reference frequency (Rf) in Hz,
Ty is a constant, and
Tc is the time since f was last computed.